Urinary tract infection
Description
An in-depth report on the causes, diagnosis, treatment, and prevention of urinary tract infections.
Alternative Names
Cystitis; UTI
Causes
The bacterial strains that cause UTIs include:
-
Escherichia (E.) coli
is responsible for 75 - 90% of uncomplicated cystitis cases in younger women and in more than half the cases in older women (over age 50). In most cases of UTI,
E. coli,
which originates as a harmless microorganism in the intestines, spreads to the vaginal passage, where it invades and colonizes the urinary tract. Some bacteria may be able to invade into deeper tissue in the bladder, where they survive to reinfect the patient after resolution of the previous infection.
-
Staphylococcus saprophyticus
accounts for 5 - 15% of UTIs, mostly in younger women. Infections caused by this bacterium tend to have a seasonal variation, with a higher incidence in the summer and fall than in the winter and spring.
-
Klebsiella
,
Enterococci
bacteria, and
Proteus mirabilis
account for most of remaining bacterial organisms that cause UTIs. They are generally found in UTIs in older women.
-
Rare bacterial causes of UTIs include
ureaplasma urealyticum
and
Mycoplasma hominis
, which are generally harmless organisms.
Organisms in Severe or Complicated Infections
-
The bacteria that cause kidney infections (
pyelonephritis
) are generally the same bacteria that cause cystitis. There is some evidence, however, the
E. coli
strains in pyelonephritis are more virulent (able to spread and cause illness).
-
Complicated UTIs that are related to physical or structural conditions are apt to be caused by a wider range of organism.
E. coli
is still the most common organism, but others have also been detected, including
Klebsiella
,
P. mirabilis
, and
Citrobacter
.
-
Fungal organisms, particularly
Candida
specie
s.
(
Candida albicans,
for example, causes the so-called "yeast infections" that also occur in the mouth, digestive tract, and vagina.)
-
Other bacteria associated with complicated or severe infection include
Pseudomonas aeruginosa
,
Enterobacter,
and
Serratia
species; gram-positive organisms, including
Enterococcus
species; and
S. saprophyticus
.
Bacterial Strains in Recurrent UTIs
Recurring infections are often caused by different bacteria than those that caused a previous or first infection.
Even if the reinfecting bacterium is still
E. coli
, it may be a variant of the original infecting
E. coli
strain. Such strains produce substances, such as one called
P fimbriae
, which tend to make the bacteria more infectious. Uncommon causes of reinfection include
Ureaplasma
and
Mycoplasma hominis,
which are sometimes associated with the acute urethral syndrome.
Factors in Overcoming the Bacterial Defense Systems
The bacteria that cause most UTIs are very common and nearly everyone harbors them. It is not clear how they proliferate and break down the natural defenses of the body. Among the possible ways this occurs are:
Changes in the Acid-Alkaline Balance of the Urinary Tract.
Changes in the amount or type of acid within the genital and urinary tracts are major contributors to lowering the resistance to infection. For example, beneficial organisms called
lactobacilli
increase the acidic environment in the urinary tract. Reductions in their number (which, for example, occurs with estrogen loss after menopause),
increases
pH and therefore the risk of infection.
Biofilm.
One theory, called the biofilm mode of growth, suggests that sometimes bacteria form capsules that adhere to the urinary tract, which protects them from many of the body's normal defenses.
-
Review Date: 6/16/2006
-
Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital
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